In recent years, with an increase in speed of personal computers and workstations, the operation speed of a computing LSI (Large Scale Integrated circuit) such as a CPU (Central Processing Unit), a DSP (Digital Signal Processor) and the like steadily has been increased. Such LSI generates more heat at a higher operation speed, i.e., a higher clock frequency. The heat from the LSI may lead the LSI itself to a thermal runaway or may affect peripheral circuits. Therefore, it is very important to appropriately cool a heat generating element including the LSI. An example of a technique for cooling the LSI may include an air cooling method using a cooling fan. In this method, for example, the cooling fan is installed to face the surface of the LSI and blows cold air onto the surface of the LSI.
FIG. 1 is a circuit diagram of a motor driving apparatus 200r. The motor driving apparatus 200r drives a single-phase DC motor 102 (also simply referred as a motor or a single-phase motor). A hall element 104 is provided in the vicinity of the single-phase motor 102. The hall element 104 generates a pair of hall signals VH+ and VH− having the opposite phases which indicate the position of a rotor of the motor 102. A hall comparator 202 compares the hall signals VH+ and VH− and generates a hall detection signal S1 indicating the position of the rotor. A logic circuit 204 transitions an H bridge circuit 206 among a plurality of states in synchronization with the hall detection signal S1, i.e., in synchronization with the rotation of the rotor of the single-phase motor 102.
The H bridge circuit 206 is connected to a motor coil 103 to be driven. When an external power supply 106 having the opposite polarity is connected to the H bridge circuit 206, a large current flows. In order to prevent such large current flow, a protection diode 212 for preventing a reverse connection is interposed between the H bridge circuit 206 and a power supply terminal VDD.
FIGS. 2A and 2B are operation waveform diagrams of the motor driving apparatus 200r illustrated in FIG. 1. FIG. 2A shows a case where the phase of a coil current IL leads and FIG. 2B shows a case where the phase of the coil current IL lags.
The motor driving apparatus 200r of FIG. 1 controls its outputs OUT1 and OUT2 according to the hall detection signal S1 irrespective of the phase of the coil current IL. Under this control, if the phase of the coil current IL lags, the coil current IL flows into the motor coil 103 from OUT2 to OUT1 in an interval during which a low-side transistor ML2 is turned on and other transistors MH1, MH2 and ML1 are turned off. At this time, the coil current IL flows into the external power supply 106 via a path 112 including the low-side transistor MH2, the motor coil 103 and a body diode of the high-side transistor MH1. As a result, as shown in FIG. 2B, there is a danger that a voltage of the output OUT2 or a power supply voltage jumps. If this jumping voltage exceeds the breakdown voltage of an IC, it has an adverse effect on reliability of the IC.
It has been conventionally common to dispose a Zener diode and a smoothing capacitor in parallel to the H bridge circuit 206 in order to suppress the jumping voltage.
In addition, the efficiency and torque characteristics of the motor are dependent on a current phase and an excessive lag of the current phase degrades the efficiency and torque characteristics.